(1) Technical Field
The present invention relates generally to frequency standards. More specifically, the present invention relates to a method and apparatus for a solid-state atomic frequency standard based on the hyperfine spectrum of paramagnetic dopants in solids.
(2) Background
Frequency standards are important in nearly all electronic systems, and are widely used in cellular telephones, wireless networks, satellite broadcasting, fiber-optic communication, global positioning systems (GPS), and a variety of general applications such as frequency synthesizers. Crystal oscillators are traditionally used as frequency standards and timing functions in all but the most demanding applications which demand precise accuracy. In addition to high accuracy, aging is also an important limitation of crystal oscillators in high-precision applications. Therefore, in demanding applications, atomic clocks, such as rubidium standards, may be used.
The cost of many high-volume electronic systems has dropped dramatically in the last several decades as greater functionality has been integrated onto a single chip. As a result, crystal oscillators are now one of the largest and most expensive components in many systems. Therefore, frequency standards with lower cost, smaller size, or better performance than the prior art have many uses.
This disclosure describes a new frequency standard. Previously, there were two classes of frequency standards: atomic clocks and crystal oscillators. In a crystal oscillator, the resonant frequency depends primarily on the precise cut, mass, and dimensions of the crystal resonator. There are various, common techniques for stabilizing frequency over temperature, such as temperature-compensated crystal oscillators (TCXOs), digitally-compensated crystal oscillators (DCXOs), time-compensated clock oscillators (TCCOs), oven-controlled crystal oscillators (OCXOs), and others known to those familiar with the art. However, the resonator need not be made of a crystal such as quartz; instead an oscillator using a surface acoustic wave (SAW) resonator or bulk acoustic wave (BAW) resonator might be used as a frequency reference in some applications. These resonators can be made from piezoelectric materials such as zinc oxide, aluminum nitride, polysilicon and others. The operating principle, however, is the same in all cases; e.g. the initial frequency is set by the mechanical dimensions of the piezoelectric resonator, and the resonant frequency is stabilized over environmental variations using techniques such as temperature compensation, as mentioned above. Also, in some cases it is desired to “pull” the oscillator to a more useful or repeatable frequency, by using a varactor in parallel with the piezoelectric resonator or by other means.
Atomic clocks derive a frequency from the quantum mechanical transitions of an ensemble of atoms or ions in the gas phase. Examples range from primary standards such as the cesium beam atomic clock, to smaller clocks such as rubidium standards. Usually, a technique is devised to interrogate the frequency of the magnetic dipole transitions between the hyperfine states of a particular atom or ion vapor, although a standard based on an optical (i.e., electric dipole) transition has been recently implemented. Rubidium standards can be implemented using optical pumping or coherent population trapping to interrogate the hyperfine splitting of the rubidium ions. Regardless of the implementation, the desired frequency is derived from the atomic spectrum of a gas of atoms or ions, rather than from the mechanical dimensions and mass of a piezoelectric resonator. Further details of conventional, miniature atomic clocks may be found in the following references, each of which is incorporated herein by reference:    {1} U.S. Pat. No. 6,426,679, entitled “Miniature, Low Power Atomic Frequency Standard with Improved RF Frequency Synthesizer,” Jul. 30, 2002;    {2} “Coherent Population Trapping for the Realization of a Small, Stable, Atomic Clock,” Jaques Vanier, 2002 IEEE International Frequency Control Symposium and PDA Exhibition, pp. 424–434;    {3} “Performance of Small Scale Frequency Reference,” J. Kitching, S. Knappe and L. Holberg, 2002 IEEE International Frequency Control Symposium and PDA Exhibition, pp. 442–446;    {4} U.S. Pat. No. 6,265,945 “Atomic Frequency Standard Based on Coherent Population Trapping,” Jul. 24, 2001; and    {5} U.S. Pat. No. 6,255,647 “Atomic Frequency Standard Based on Coherent State Preparation,” Jul. 3, 2001.
The following articles describe the frequency stability of passive atomic frequency standards, and are incorporated herein by reference: Jacques Vanier and Laurent-Guy Bernier, “On the Signal-to-Noise and Short-Term Stability of Passive Rubidium Frequency Standards,” IEEE Transactions on Instrumentation and Measurements, (1981), IM-30, 277; Jacques Vanier, Michel Tetu, Laurent-Guy Bernier, “Transfer of Frequency Stability from an Atomic Reference to a Quartz-Crystal Oscillator,” IEEE Transactions on Instrumentation and Measurements, (1979), IM-28, 188; James A. Barnes, et al., “Characterization of Frequency Stability,” IEEE Transactions on Instrumentation and Measurements, (1971), 20, 105.
(3) Description of Related Art
Several patents are described below, which may provide further useful background material to the reader: U.S. Pat. No. 6,570,459, issued May 27, 2003, to Nathanson et al., discloses a physics package apparatus for an atomic clock. The physics package, containing a cesium or rubidium vapor, laser diode, collimating optics, photodetector, microwave resonator, and heating elements, is batch fabricated using laminated layers of etched borosilicate glass.
U.S. Pat. No. 6,133,800, issued Oct. 17, 2000, to Jinquan Deng, discloses a subminiature microwave cavity which is used to implement a small and inexpensive physics package for an atomic clock. The cavity is a variation on a capacitive-loaded coaxial resonator.
U.S. Pat. No. 4,803,624, issued Feb. 7, 1989, to Pilbrow et al., discloses a portable electron spin resonance spectrometer.
U.S. Pat. No. 3,548,298, issued Dec. 15, 1970, to M. S. Adler, discloses a transistorized nuclear magnetic resonance gaussmeter. The gaussmeter is a lock-in nuclear magnetic resonance gaussmeter, i.e. a system is disclosed to servo the frequency of a RF source to the frequency of a nuclear magnetic resonance absorption.